everyone should read savage love anyway.
there are more examples but i cant be bothered to go through his entire archive right now (though i have read almost his entire archive, from the bottom, very amusing, educational, good for sexual emotional intelligence and v. interesting re: sex politics in america)

It's inappropriate, but not in an offensive sense. It's more that it's a concept that you needn't really apply to infidelity. The word justification carries too much baggage with it. Of course you can explain 'good'(presuming that we've all agreed on an absolute moral stance on cheating) reasons for cheating on a partner and these reasons can resonate with people quite well. I don't think Savage is really blazing a trail with that idea.

what he's described isn't cheating. It's sleeping around with others, with it being discussed in advance with your partner. But it's not cheating. Furthermore, I'd wager that such an arrangement is not likely to be a particularly easy or long-lived arrangement in the most part for most folk.